1. Field of the Invention
The present invention relates to a deposit removing method and a substrate processing method, and in particular relates to a deposit removing method in which deposit resulting from plasma produced from a process gas containing methane gas and nitrogen gas.
2. Description of the Related Art
In general, plasma etching processing in which trenches, holes, and so on are formed in a semiconductor wafer is carried out in an evacuated processing chamber in which a semiconductor wafer is accommodated. In the processing chamber, a mounting stage on which the semiconductor wafer is mounted, and a showerhead that supplies a process gas into the processing chamber are disposed. The mounting stage is connected to a radio frequency power source and acts as an electrode that applies radio frequency voltage to the interior of the processing chamber. The mounting stage produces an electric field in the processing chamber, and the process gas is turned into plasma by the electric field.
If the plasma etching processing is carried out on the semiconductor wafer using plasma produced from a certain type of process gas, reaction product is produced and becomes attached as deposit to component parts in the processing chamber, for example, a showerhead.
If deposit becomes attached to the showerhead, it becomes difficult to stably produce an electric field in the processing chamber, and therefore, plasma cannot be stably produced. As a result, the etch rate for the semiconductor wafer may decrease as the time period for which radio frequency voltage is applied passes as shown in FIG. 7.
As a method of removing such deposit comprised of reaction product, there is known a method in which plasma is produced from a mixed gas containing oxygen gas (O2 gas), and deposit is removed by the plasma (see, for example, Japanese Laid-open Patent Publication (Kokai) No. H08-279487). In particular, there is known that deposit resulting from plasma etching processing using plasma produced from a process gas containing CF-based gas can be efficiently removed by ashing using plasma produced from a mixed gas containing oxygen gas.
In recent years, finer machining has been demanded, and a process gas containing methane (CH4) gas has been holding promise for realizing such machining.
However, the process gas containing methane gas includes nitrogen (N2) gas as well, CN-type reaction product is produced as a result of the plasma etching processing.
The CN-type reaction product becomes very tightly attached to the showerhead and so on, and it is thus difficult to remove the CN-type reaction product. For example, before developing the present invention, the present inventors used plasma produced from a single gas comprised of oxygen gas so as to remove the CN-type reaction product, and ascertained that the CN-type reaction product can be hardly removed even if ashing using the plasma (pressure: 800 mTorr, supplied radio frequency electrical power: 2000 W, oxygen gas flow rate: 1200 sccm, and magnetic flux density: 300 G) is carried out for 30 seconds. The present inventors also ascertained that the CN-type reaction product can be hardly removed even if plasma produced from a mixed gas comprised of oxygen gas with CF4 gas having high sputtering power added thereto is used.